Increase the max upload filesize in apache/php
Edit your php.ini. where is my php.ini?
; Maximum allowed size for uploaded files.
upload_max_filesize = 2M
Change the 2M
to whatever new value you want, then restart apache.
Note, this setting may also be related:
; Maximum size of POST data that PHP will accept.
post_max_size = 8M
Note, some RoundCube versions have limits set in the file
so be sure to increase them there as well, if they're set.